Since a sintered body of a ceramic such as alumina, silicon nitride, or aluminum nitride exhibits excellent heat resistance, wear resistance and electrically insulating performance, it is widely used as a semiconductor substrate, automotive part, etc. When a sintered ceramic body is applied to a power semiconductor module, engine part, or external wall of a rocket or airplane, it should additionally have high heat-radiating capability. However, generally, a sintered ceramic body suffers from low thermal conductivity. In such cases, therefore, a metallic body (e.g., copper sheet) having high thermal conductivity is bonded to a sintered ceramic body, for increasing the heat-radiating capability of the ceramic body.
Meanwhile, since a sintered body of a ceramic such as alumina, aluminum nitride oxide, aluminum nitride, or boron nitride, or a composite body of two or more of those sintered ceramic bodies exhibits relatively high thermal conductivity and electrically insulating performance, among various sintered ceramic bodies, and additionally has excellent mechanical properties, a circuit pattern in the form of a metal sheet such as copper sheet is bonded to the sintered ceramic body or composite ceramic body so as to provide a composite structure as a semiconductor-mounting substrate on which a heat-generating device such as an IC package or power semiconductor is mounted.
The above composite structure is produced, e.g., by a process wherein a metallic body is brazed to a sintered ceramic body using, e.g., a silver brazing filler, or by a process wherein first a sintered ceramic body is held in contact with a metallic body and subsequently the two bodies are bonded to each other by eutectic reaction caused in an appropriate atmosphere as disclosed in Japanese Patent Application laid open for opposition under No. 57(1982)-13515.
However, the coefficient of thermal expansion of a commonly used sintered ceramic body falls within the range of 3.times.10.sup.-6 to 10.times.10.sup.-6 /.degree.C., whereas metal such as copper or stainless steel has a thermal expansion coefficient falling within the higher range of 16.times.10.sup.-6 to 18.times.10.sup.-6 /.degree.C. Thus, the above-described composite structure has a large difference between the thermal expansion coefficients of the sintered ceramic body and the metallic body bonded to each other. Therefore, the composite structure suffers from stresses (i.e., thermal stresses) caused by that difference, when the composite structure is subjected to temperature changes in the bonding step or during its service. Those thermal stresses may cause peeling at the interface of bonding of the two bodies, or may cause cracks in the sintered ceramic body if the mechanical strength of the ceramic body is lower than the stresses. For overcoming the those problems, it has been practiced to provide, at the bonding interface of a sintered ceramic body and a metallic body, a buffer material which has a thermal expansion coefficient intermediate between those of the two bodies. Meanwhile, Japanese Non-Examined Patent Application laid open under No. 56(1981)-41879 discloses the technique of interposing, between a sintered ceramic body and a metallic body, a buffer material which has high ductility and therefore is plastically deformable largely enough for absorbing the thermal stresses generated between the two bodies. However, in the above-indicated first technique, there still remains a considerable difference between the thermal expansion coefficients of the buffer material and the sintered ceramic body, so that thermal stresses due to that difference are produced in the composite structure. Meanwhile, in the above-indicated second technique, the plastic deformation of the buffer material may not be so large as to absorb the thermal stresses generated, if those stresses are very large.
The present invention has been developed in the above-described background. It is therefore an object of the present invention to provide a ceramic-metal composite structure free from problems such as peeling of a metallic member from a sintered ceramic body, or generation of cracks in the sintered ceramic body, each due to temperature changes.